In a casting process, the liquid metal needs at one point to be transferred from an holding furnace to the molds of a casting pit, where it is poured into the molds and cooled to make ingots or billets. The transfer of liquid metal from the holding furnace to the molds is generally made using an opened or closed channel called a launder. A launder is also called a supply gutter in some other references. A launder may further be used for transferring liquid metal from an alloying furnace, if any, to the holding furnace. As its name indicates, an alloying furnace is used for combining various metals together in the required proportions so as to prepare alloys.
The exterior walls of a launder are usually made of mild steel and constitute the frame thereof. The interior side of the frame is generally lined with a layer of compacted ceramic wool or another kind of resilient and high-temperature resistant insulating material. The portion of the launder in contact with the liquid metal is typically made of a solid refractory material. The refractory material is used to reduce the heat losses and to prevent the pick-up of contaminating materials.
The holding furnace typically contains several tons of liquid metal which need to be transferred to the casting pits over a period of time ranging from a few hours in the case of a semi-continuous process, to many consecutive days in the case of a continuous process. A key factor for the full success of a casting operation is the uninterrupted and constant supply of liquid metal during the transfer. If the metal stops from flowing or if the flow rate changes while the casting operation is under way, appropriate actions and corrective measures have to be taken immediately. As a result, the transfer and casting operation require that the level of liquid metal flowing through the launder be measured and monitored in a reliable and accurate fashion. There is thus a need for a system to continuously monitor the level of liquid metal in a launder so as to ensure that the proper amount is continuously flowing.
While some prior attempts to provide devices for measuring the level of liquid metal in a launder have resulted in a number of different constructions, none has been found completely satisfactory. For instance, some systems use a laser beam to measure the reflectivity of the launder and its contents. These systems use the surface reflectivity to obtain a signal back from the liquid metal and to measure the level thereof. However, when the surface is too shiny or when the dross is too thick, the signal is lost. Similar problems may happen when dense fumes obscure the region above the launder. Some other systems use the electrical capacitance of the launder and its contents. However, there is a suitable response only for a few inches and this is generally not sufficiently precise nor accurate in many applications. There is also a low signal to noise ratio, making it difficult to obtain an accurate value of the liquid metal level.
The object of the present invention is to reduce the difficulties and disadvantages experienced with prior systems by providing an improved system and a method for measuring liquid metal levels in a launder or any similar locations where such measurements need to be undertaken. An important aspect of the present invention is that it is not significantly affected by the presence of steel on the exterior side of the launder. It is further stable in the harsh environment of a cast house and may work even if there is no external cooling.
The full extent of the present invention will be more readily apparent from the following detailed description of preferred embodiments thereof, which proceeds with reference to the accompanying figures.